FIELD OF THE INVENTION

The invention relates to the field of illumination devices and methods for camera applications, such as for illuminating a field of view of a camera for surveillance or observation purposes.

BACKGROUND OF THE INVENTION

Camera observation is omnipresent, and is more and more being used for surveillance of traffic, crowds, and security-relevant industry sites. In many cases, the cameras are equipped with near-infrared illuminators, to be able to observe during nighttime or other low light level conditions as well. Usually light between 760 and 940 nm is used, since the human eye is hardly or not at all sensitive to light at this wavelength, whereas silicon, of which the light-sensitive camera pixels are made, still has sufficient quantum efficiency.

In some cases the camera is directly equipped with near-infrared (NIR) illuminators (which are often based on light emitting diodes (LEDs), and in other cases (halogen) lamps are installed on light poles around the observation area.

Illuminators, especially when mounted close to the camera, illuminate the field of view (FOV) of the camera. The illuminator itself radiates uniformly or almost uniformly into the scene. Due to spreading of the light beam (in connection with the FOV), the light of the illuminator is spread over a larger area, the larger the travel distance. As a result, the radiance of the objects illuminated by the illuminator decreases, and hence objects at a larger distance will appear darker in the camera image. For example, in case of a front-lid camera, objects that are close to the camera appear much brighter in the picture. In many cases, a small diaphragm is required to avoid over-exposure, which is disadvantageous in that sensitivity to light of the camera is not fully exploited, thus spilling light of the illuminator and rendering sub-optimal images, in which the background or other objects located further away from the camera are not visible.

Fig. 1 shows a schematic diagram indicating actual light levels of a conventional illuminator 310 on ground 300 over a distance of 100 m from the illuminator 310. As the luminous flux travels away from the illuminator 310, the area over which it spreads increases, therefore the illuminance or light intensity (lux) must decrease. The relationship is expressed by an inverse square law. The ratio from 100 m to 10 m is thus 100: 1. The illustration in Fig. 1 is for a white light because infrared light cannot be measured in lux. The effect of this law though, affects infrared light in exactly the same way.

In addition, an object that is located off-axis, close to the maximum FOV of a camera appears darker in the image, even if the radiance of the object is identical to the object on axis. The effect, known as the cos ⁴ law, has several causes, among others the solid angle of the camera decreases, and the magnification off-axis increases. For this reason, even without a common problem of vignetting (which is an effect of lens aperture), objects at the border of the image appear darker.

Conventional illuminators or lamps which circumvent this problem all employ optical means (such as micro-refractive lens technology), ending up with large and bulky systems, and non-conventional elements.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved illumination device and method, by means of which the sensitivity of the camera can be fully used while avoiding over-exposure.

This object is achieved by an array as claimed in claim 1, an illumination device as claimed in claim 7, by a method as claimed in claim 11 and by a computer program product as claimed in claim 13.

Accordingly, the array is adapted so that the density or aperture or both of the radiation elements changes in at least one portion of the array, or the emission power of individual ones or clusters of the radiation elements is controlled based on the output image and/or the diaphragm settings of the camera device so as to provide an adapted distribution of radiation. Light distribution of the array can thereby be configured so that less light is sent to brighter objects close to the camera. By this measure the diaphragm of the camera can be kept maximally open during every image capture, ensuring that the sensitivity of the camera is always fully used, and no light and thus electrical energy is spoiled.

According to a first aspect, the array may be divided into different zones arranged one after the other along a dimension of the array, wherein each zone has a different density of the radiation elements so that the density decreases or increases in a stepwise manner along said dimension of the array. Thereby, the radiation power projected on the illuminated area can be stepwise increased or decreased, respectively, along the projection of the dimension. According to a second aspect which can be combined with the above first aspect, the array may be divided into different zones arranged one after the other along a dimension of the array, wherein each zone has radiation elements with a different aperture size, so that the aperture size decreases or increases in a stepwise manner along the dimension of the array. Similar to the first aspect, the radiation power projected on the illuminated area can be stepwise increased or decreased, respectively, along the projection of the dimension.

According to a third aspect which can be combined with the above first or second aspect, the array may be provided in an illumination device, wherein a lens may be arranged in front of the array and adapted to project the radiation of the radiation elements to predetermined angles, e.g, of an area to be illuminated. Thereby, the illumination device can be enhanced to provided an adapted illumination characteristic.

According to a fourth aspect which can be combined with the third aspect, the illumination device may comprise a switched transistor matrix for controlling current supplied to the radiation elements of the array. The switched matrix allows an individual voltage-based control of the current flowing through each of the radiation elements and thus the radiation output power of each radiation element. In a modification of the fourth aspect, three-dimensional information of a scene of the field of view may be derived from a matrix of controlled currents supplied to the radiation elements.

According to a fifth aspect which can be combined with any one of the first to fourth aspects, the emission power of the radiation elements may be adjusted so that bright areas of the field of view of the associated camera device receive less radiation power and dark areas of the field of view receive more radiation power, if the diaphragm settings indicate that the diaphragm of the camera device is not fully open. Thereby, it can be ensured that the field of view is illuminated in a manner so that the camera diaphragm can be kept fully open while overexposure is prevented.

The above array of radiation elements can be implemented as an integrated or monolithic chip or chip module or chip-set which may be distributed or supplied separately so as to be integrated into various types of illumination devices. The control unit of the illumination device may be implemented by a computer or signal processing device or chip controlled by a software routine or program stored in a memory, written on a computer readable medium, or downloaded from a network, such as the Internet. Thus, the software routine or program may comprise program code (i.e. code means) for producing the steps of method claim 11 when run on the computer or signal processing device. It shall be understood that the array of claim 1 , the illumination device of claim 7, the method of claim 11 and the computer program product of claim 13 have similar and/or identical preferred embodiments, in particular, as defined in the dependent claims.

It shall be understood that a preferred embodiment of the invention can also be any combination of the dependent claims with the respective independent claim.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Fig. 1 shows a schematic diagram indicating actual light levels of a

conventional illuminator over a predetermined distance;

Fig. 2A shows a schematic cross-sectional side view and light output

characteristic of an illuminator device according to a first

embodiment with different radiator density;

Fig. 2B shows a schematic top view of the illuminator device according to the first embodiment;

Fig. 3 shows different zones of light projected by the illuminator device of the first embodiment at different distances;

Fig. 4 shows a schematic top view of an illuminator device according to a second embodiment with different radiator aperture;

Fig. 5 shows a characteristic of voltage versus current for various radiator elements with different apertures;

Fig. 6 shows a characteristic of output power versus current for various radiator elements with different apertures;

Fig. 7 shows a schematic top view of an illuminator device which can be used in a third embodiment;

Fig. 8 shows a schematic diagram indicating optical paths of a radiator array imaged with a positive lens to a field of view;

Fig. 9 shows a schematic block diagram of an illumination control system according to the third embodiment; and

Fig. 10 shows a schematic circuit diagram of a parallel circuit of controllable radiation elements according to the third embodiment. DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments are now described based on an illumination device with an array of vertical-cavity surface-emitting lasers (VCSELs) which are a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane lasers) which emit from surfaces formed by cleaving the individual chip out of a wafer. Because VCSELs emit the beam perpendicular to the active region of the laser as opposed to parallel as with an edge emitter, tens of thousands of VCSELs can be processed simultaneously on a three inch Gallium Arsenide (GaAs) wafer. Furthermore, even though the VCSEL production process is more labor and material intensive, the yield can be controlled to a more predictable outcome. Light output of the VCSEL array is provided by mesa structures. A mesa is an elevated area of land with a flat top, surrounded on all sides by steep cliffs. A semiconductor structure with a shape similar to this geological formation is called mesa structure.

According to some embodiments, VCSEL array are modified or controlled, so that they can solve irregularities in apparent brightness of an object in a camera image. To achieve this, the mesa density and/or the mesa aperture is increased in some parts of the array which may be provided on a chip. Additional optics (e.g. lens design) may be provided to ensure that the light of these areas is project to angles which reach further in the scene. As an additional or alternative measure, a control unit may be provided which analyses camera images and the diaphragm setting of the camera, and allows an adjustment of the VCSEL array of the illuminator such that less light is sent to objects close to the camera.

In the embodiments, the VCSEL array may be used as a NIR illuminator, where the array is designed such that single emitters on the chip or clusters thereof can be controlled externally, e.g., by the control unit. The array may consist by design of thousands (>2000 for a 4 mm chip) single emitters (i.e. radiation elements or radiators). With proper optical design, light from neighboring emitters is mixed, yet small clusters of emitters are projected to different angles in the scene. In this way, more light can be sent to background regions of an illuminated area (e.g., field of view of the camera), illuminating the background strongly for a more even brightness in the whole image. By proper adaption of the light distribution of the array less light is sent to objects close to the camera. It can thus be ensured that a diaphragm of the camera is maximally open during every image capture, ensuring that the sensitivity of the camera is always fully used, and no light and thus no electrical energy is wasted. Fig. 2A shows a schematic cross-sectional side view and light output characteristic of an illuminator device according to a first embodiment with different density of radiators (i.e. VCSEL mesas) 10 of a VCSEL array 100 (VCSEL chip) and with three different zones A, B and C arranged one after the other in the x-direction indicated in Fig. 2A. Zone A has a higher filling ratio, and thus more active (emitting) area per chip area. Zone B has less active area density, and zone C again less. Thus, the density and light intensity (I) is stepwise decreased in the x-direction along the areas A, B and C, as indicated in the lower left diagram of Fig. 2A. After a certain distance (indicated by the vertical line between the VCSEL array 100 and a lens 200) away from the chip with the VCSEL array 100, the light from neighboring lasers of VCSELs will start to mix. At even further distance light of next- neighboring lasers mix. At this distance, the point pattern of the chip is not visible anymore, since enough light mixing has taken place. On the other hand, the zones A, B and C will have different brightness, since light from different zones has hardly mixed. If this plane is imaged by the simple positive lens 200 with focal length L, the same profile in illumination can be projected on the scene, cancelling out e.g. distance effects as discussed above. Due to the lens 200, light of upper radiators in Fig. 2A is projected to lower portions of the field of view. The lower right diagram of Fig. 2A shows a steps wise decrease of light intensity along an elevation angle Θ.

Fig. 2B shows a schematic top view of the illuminator device according to the first embodiment. In this example, the zones A, B and C cover the whole width of the array 100 in the horizontal direction of Fig. 2B (i.e. z-direction in Fig. 2A).

Fig. 3 shows different zones of light projected by the illuminator device of the first embodiment of Figs. 2A and 2B at different distances on a ground plane 300, where light from zone A with the highest emitter density travels the longest distance from the VCSEL array 100 and lens 200 to the ground plane. Thereby, with suitable settings, a constant level of brightness can be achieved throughout the three projected zones of light on the ground plane 300.

Fig. 4 shows a schematic top view of an illuminator device with a VCSEL array 100 with different radiator apertures according to a second embodiment. In the second embodiment, the radiator aperture (i.e. laser aperture of the VCSEL) is varied over the chip area of the VCSEL array 100, instead of the radiation density (i.e. laser mesa density). This has as advantage that the chip size can be kept small. Thus, VCSEL emitters (or mesas) 12, 14 with different aperture sizes (e.g. aperture diameters) are provided in the array 100. More specifically, the apertures are constant along the horizontal direction or rows of the VCSEL array 100, while they change along the vertical direction or columns of the VCSEL array 100. In the specific example of Fig. 4, the emitters 12 on the upper row of the VCSEL array 100 have the smallest aperture and the emitters 14 in the lowest row of the VCSEL array 100 have the largest aperture.

Furthermore, a p-contact area 20 of the VCSEL chip is shown in Fig. 4. Since the VCSEL emitters 12, 14 (i.e. mesas) share the same p-contact 20 (top) and n-contact (bottom, not shown), they are connect in parallel, and will operate at an identical laser voltage.

In the next two Figs. 5 and 6 characteristic curves of single devices with different apertures are shown.

Fig. 5 shows a characteristic of voltage versus current for various radiator elements (i.e. VCSELs) with different apertures (values 4μιη, 6 μιη, 8 μιη and 10 μιη). As can be gathered from Fig. 5, lower currents are drawn by radiator elements with smaller apertures.

Fig. 6 shows a characteristic of output power versus current for the various radiator elements with different apertures of Fig. 5. As can be gathered from Fig. 6, higher optical output powers can be achieved with radiator elements with larger apertures.

Fig. 7 shows a schematic top view of an illuminator with a controllable VCSEL array 110 with VCSEL emitters 10, which can be used in a third embodiment where images and diaphragm setting of a camera are analyzed, and the VCSEL emitters 10 of the illuminator are adjusted. The illuminator may be an NIR illuminator and the VCSEL array 110 is designed such that single VCSEL emitters 10 or clusters thereof on the illuminator chip can be controlled externally. Again, the VCSEL array 110 may consist by design of thousands (>2000 for a 4 mm chip) single VCSEL emitters 10. With proper optical design, light from neighboring emitters is mixed, yet small clusters of emitters are projected to different angles in the scene. In this way, more light can be sent to be background, illuminating the background strongly for an even brightness in the whole image. By proper adaption less light is sent to objects close to the camera. Thereby, it can be ensured that the diaphragm is maximally open during every image capture, ensuring that the sensitivity of the camera is always fully used, and not light and thus electrical energy is spoilt.

Fig. 8 shows a schematic diagram indicating optical paths of a radiator array imaged with a positive lens 200 to a field of view. In Fig. 8 the optical paths of two different exemplary emitters 1, 2 of the VCSEL array 110 of Fig. 7 are shown. It is clear that radiation beams LI of the emitter 1 on top of the array are emitted or projected via the lens 200 to the lower part of the illuminated scene after the focal length f, whereas light beams L2 of the emitter 2 on the bottom of the array are sent to the top of the illuminated scene. Thus, if emitters on the top portion of the array are controlled to increase their output power, illumination of lower portions of the illuminated scene will be stronger and they will appear brighter to the camera.

Fig. 9 shows a schematic block diagram of an illumination control system according to the third embodiment. A camera 500 with an optical system (e.g. lens) 600 takes pictures of a scene which is artificially lit by an illuminator with a VCSEL array 110 and a lens 200 or other optical system. The camera 500 is configured to automatically adapt its diaphragm 700 so as to avoid overexposure of the scene in its field of view. A control unit 400 (e.g. a software-controlled microcontroller or microcomputer) analyses the image taken by the camera 500 and diaphragm settings of the camera 500. If the control unit 400 determines, based on the diaphragm settings, that the diaphragm 700 of the camera 500 is not fully opened, the control unit 400 supplies control signals or control information to the illuminator with the VCSEL array 110 so as to control or adjust the output power of the VCSEL emitters such that bright areas of the illuminated scene receive less light and dark areas receive more light. To achieve this, the illuminator may be adapted to apply less current to those VCSEL emitters projecting their light to the bright areas (e.g. close objects of the scene) and vice versa, in response to the control signal or control information received from the control unit 400.

Fig. 10 shows a schematic circuit diagram of a parallel circuit of controllable radiation elements (e.g. laser diodes such as VCSEL emitters) 10 which can be used in the third embodiment. In Fig. 10, an exemplary and simplified electrical circuit of a portion of the radiator array (e.g. VCSEL array 100) is shown. In the example, six laser diodes (e.g. VCSEL emitters) 10 are depicted and connected in parallel. By supplying adjusting currents il to i6, the different lasers diodes 10 will emit an amount of light proportional to the respective current.

In another example, the VCSEL array 100 may be combined with a switched matrix of transistors or other voltage-controlled semiconductor elements, such that the current through each laser diode (e.g. VCSEL emitter) can be controlled by a respective voltage. Assuming that all objects in the field of view of the camera have similar diffuse reflectivity at NIR wavelengths, the switched matrix example might provide (e.g., by the control unit 400 of Fig. 9) rough three-dimensional (3D) map information of the scene in the field of view. If the laser currents are arranged such that uniform illumination is reached, the matrix of currents corresponds to a rough representation of the 3D scene in front of the camera. This 3D information can be used for various purposes such as control of the camera position and/or viewing direction based on closest objects, or conversion of the 2D camera image into a 3D image.

To summarize, an illumination device and method of illuminating a predetermined field of view have been described, wherein an array of radiation elements solves irregularities in apparent brightness of an object in a camera image by providing an adapted distribution of radiation towards the field of view. This is achieved by increasing or decreasing density and/or aperture of radiation elements in some parts of the array or by controlling emission power of single radiation elements or clusters thereof. By a suitable design of a lens in front of the array it can be ensured that the radiation of these radiation elements is projected to angles which reach further in the scene, and thus lead to a camera image with constant brightness.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. It can be applied in any field of illumination devices or illumination arrays with all types of radiation elements. The different apertures or densities of the radiation elements may be achieved by lens systems or other optics by which the light distribution generated by the array can be modified. The control unit may be integrated in the camera or in the illuminator. Moreover, the illuminator may be integrated in the camera together with the control unit.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the invention may be practiced in many ways, and is therefore not limited to the embodiments disclosed. It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to include any specific characteristics of the features or aspects of the invention with which that terminology is associated.